The present invention pertains to the electrochemical arts, in the reduction of oxygen to hydrogen peroxide. It finds particular application in the field of concentration, and more particularly in concentrating oxygen from a gaseous mixture, such as air, according to an electrochemical device and method, and will be described with particular reference thereto. It should be appreciated, however, that the invention can be applied to other areas not set forth herein, particularly where concentrated oxygen or hydrogen peroxide is desired.
There is a need for providing substantially undiluted oxygen in a variety of medical, industrial, aeronautical, and space exploration situations. Numerous electrochemical methods and devices are known in the art for concentrating oxygen. Such known electrochemical devices and methods for generating oxygen are based on electrolysis using solid polymer electrolyte (SPE) technology, static feed H2O electrolysis subsystems (SFWES), or the less-developed electrolysis of carbon dioxide. The power consumption of these processes is often high, typically 100-200 W/kg of oxygen generated per day. Conventional electrochemical oxygen generators are based upon the electrolysis of water, a 4-electron process.
U.S. Pat. No. 5,338,412 discloses an electrochemical device and method for the selective removal and regeneration of oxygen from the ambient atmosphere. The device is based on the use of a two electrode system separated by a thin layer of liquid or solid electrolyte. An external potential difference is applied between the two electrodes to promote the reduction of oxygen to hydrogen peroxide at the cathode. The hydrogen peroxide is then reoxidized at the anode to yield gas-phase purified oxygen. Hence, if the cathode is exposed to the atmosphere and the anode is exposed to an enclosed environment, the device will selectively enrich the enclosed environment with oxygen.
The electrochemical reduction of oxygen to peroxide under alkaline conditions is a highly reversible process occurring at a high exchange current density. Isotopic studies of the cathodic reduction of O2 to HO2xe2x88x92 and the reverse anodic process on high-area porous carbons and alkaline solutions demonstrate that the oxygen-oxygen bond is not ruptured during these reactions. For instance, an Exc2x0 value on carbon for the O2/HO2xe2x88x92 couple in alkaline solutions has been determined to be xe2x88x920.048 V. Polarographic measurements on mercury provide Exc2x0=xe2x88x920.045 V.
In contrast, the electrochemical reduction of O2 to H2O or OHxe2x88x92, and the electrolysis of water under acidic or alkaline conditions, are irreversible processes. Such processes are highly dependent on the catalytic capability of the electrode material for facilitating oxygen-oxygen bond breaking (reduction) or bond making (oxidation).
Cell voltages are dependent upon the activation over-potentials at the electrodes and on the IR contributions of the separator, electrode components, solution and electrical connections. Mixed electrochemical processes, cause by inefficiencies resulting from catalytic decomposition of peroxide and O2 formation from electrolysis of water instead of peroxide, will raise the cell voltage if uncontrolled. By minimizing these inefficiencies, the overall cell voltage will be close to zero volts.
The oxygen concentration device is well suited for applications in areas where oxygen is continuously consumed in or by the enclosed environment. Power consumption is anticipated to be around 40-50 W/kg O2 per day.
For the device described in U.S. Pat. No. 5,338,412 to operate with an acidic electrolyte such as Nafion, the cathode should be compatible with the acid media. Few materials have been found which are capable of catalyzing the reaction of oxygen to hydrogen peroxide in such acidic conditions. Strbac et al. disclose the conversion of oxygen to hydrogen peroxide on various crystal faces of gold, including Au(100), Au(311), and Au(210), in acidic electrolytes (S. Strbac and R. Adzic, J. Serbian Chem. Soc. 57 (12) 835-848 (1992)). They found the highest activity on Au(100) in sulfuric acid (pH 1), where the onset of the reaction was at about 0.6V vs a standard hydrogen electrode(SHE). However, few of the single crystal faces of gold show a high specificity for the conversion of oxygen to hydrogen peroxide. This lack of specificity makes the formation of a high surface area gold catalyst difficult since it is extremely hard to achieve the preferred crystallographic orientation over the entire surface.
The present invention provides a new and improved method and apparatus for the selective conversion of oxygen to hydrogen peroxide in acidic media which overcomes the above-referenced problems, and others. The subject new electrode and method for its application may lead to inexpensive sources of pure oxygen for medical uses and, on a larger scale, for industrial applications such as steel production or glass manufacturing.
In accordance with one aspect of the present invention, an electrode which is selective for reduction of oxygen is provided. The electrode comprises a substrate which includes platinum, and is characterized by the substrate having a surface modified with a layer of a substance selected from the group consisting of sulfur, selenium, and tellurium, compounds of sulfur, selenium, and tellurium, and combinations thereof.
In accordance with another aspect of the present invention, an electrochemical device for concentrating oxygen from a feed gas including oxygen is provided. The electrochemical device comprises a cathode for reducing oxygen in the feed gas to peroxide, an anode for oxidizing the peroxide to oxygen, a separator between the anode and the cathode for selectively diffusing the peroxide therethrough, and a source of a potential difference connected with the cathode and the anode. The device is characterized by the cathode including a platinum-containing substrate which has a surface modified with a substance selected from the group consisting of sulfur, selenium, and tellurium, compounds of sulfur, selenium, and tellurium, and combinations thereof.
In accordance with another aspect of the present invention, a method of converting oxygen in an oxygen-containing atmosphere to peroxide with a high degree of specificity is provided. The method is characterized by modifying the surface of a platinum-containing electrode with a substance selected from the group consisting of sulfur, selenium, and tellurium, compounds of sulfur, selenium, and tellurium, and combinations thereof, and contacting the electrode surface with the atmosphere.
One advantage of the present invention is that it enables the conversion of oxygen to hydrogen peroxide in acidic electrolytes.
Another advantage of the present invention is that it provides for an oxygen concentration device having a low power consumption at higher rates of oxygen production.
Yet another advantage of the present invention is that the electrode is highly selective for the conversion of oxygen to hydrogen peroxide, even when the electrode is partially deactivated.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.